ISBSG

The International Software Benchmarking Standard Group (ISBSG) is a not-for-profit organization that maintains two repositories of software development metrics. One is for Software Development and Enhancement and the other is for Software Maintenance and Support.
There are several ways to estimate Work Effort using the data from the ISBSG repository. Three common ways are by using Regression Equations, using Project Comparison, or by using Project Analogy.
Regression Equations generate a ballpark estimate by plugging values into specific equations. The estimates generated though are not accurate enough to be used for actually managing the project.
With Project Comparison the ISBSG data is mined for projects that have similar FP size (expressed as a range) as the target project. The resulting projects attributes are then compared to the target project to find those that are somewhat similar to the target project. These attributes include the language used, application type, architecture, methodology used, etc. For each of these projects the optimistic, most likely, and conservative values for the project delivery rate (PDR) and speed of delivery (SD) are calculated. The average of the PDR and SD are calculated to determine the target project delivery rate (PDRCE) and the speed of delivery (SDCE). The project work effort and project duration are then found by:

Project Work Effort PWECE = PDRCE * Project Size
Project Duration PDCE = Project Size / SDCE

Project Analogy is similar to Project Comparison with the exception that instead of finding a group of projects that match the target project a single matching project is used. The ISBSG data is mined for the project that most resembles the target project and using its actual values for PDR and SD to calculate the project work effort and project duration.
The Project Analogy method is more prone to errors in projecting work effort and duration since the odds of picking a project that does not actually match the target project is high.

Example

In Practical Project Estimation: A Toolkit for Estimating Software Development Effort and Duration, Second Edition by Peter R. Hill there is an example of using Project Analogy to estimate a projects effort.
The project used is a student and staff record management system for a university. The computed functional size of the program is 540 FP, the development type was "New Development":